Due to the superior optical and electrical properties of Group III-V materials, homogeneous epitaxial III-V solar cells are incomparable when considering energy conversion efficiency. (See, M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, Prog. Photovolt. Res. Appl. 2015, 23, 1.) However, they generally are too expensive for many terrestrial and non-concentration applications. Meanwhile, single crystal Si solar cells have been widely adopted by households and solar plants due to their affordable price. Although, their efficiency is limited by thermal relaxation of hot carriers. In order to take advantage of the benefits of both types of solar cells, numerous attempts have been made to fabricate high quality hybrid Group III-V/Si tandem solar cells using various techniques.
Direct growth is a process that can simplify the device processing flow for tandem solar cells. However, the challenge of epitaxial growth of high quality III-V materials directly on Si substrates, while long identified, remains partially unsolved. The main problem arises from the large lattice mismatch between GaAs and Si, and disparities in thermal expansion coefficients between III-V semiconductor materials and Si. In addition, the transition from non-polar Si to polar III-V materials could introduce an anti-phase domain. AlGaAs/Si tandem solar cells and GaInP/GaAs tandem solar cells on non-active Si carrier substrates have been demonstrated. (See, T. Soga, T. Kato, M. Yang, M. Umeno, T. Jimbo, J. Appl. Phys. 1995, 78, 4196 and F. Dimroth, T. Roesener, S. Essig, C. Weuffen, A. Wekkeli, E. Oliva, G. Siefer, K. Volz, T. Hannappel, D. Haussler, W. Jager, A. W. Bett, IEEE J. Photovolt. 2014, 4, 620.) However, their performance is limited by short minority diffusion lengths resulting from high dislocation densities of over 107/cm2, despite the thick buffer layers used. To circumvent the problem with heterogeneous growth of III-V on Si, a thin single crystal nucleation layer, such as an InP and GaAs layer, has been transferred onto Si solar cells by wafer bonding and lift-off. Subsequently, III-V solar cells were grown on the InP/Si or GaAs/Si solar cells. (See, J. Schone, F. Dimroth, A. W. Bett, A. Tauzin, C. Jaussaud, J. C. Roussin, in Conf Rec. 2006 IEEE 4th World Conf. Photovolt. Energy Conyers., 2006, pp. 776-779 and M. J. Archer, D. C. Law, S. Mesropian, M. Haddad, C. M. Fetzer, A. C. Ackerman, C. Ladous, R. R. King, H. A. Atwater, Appl. Phys. Lett. 2008, 92, 103503.) However, these processes not only complicate fabrication, but also suffer from the formation of cracks in the III-V thin film during growth, due to thermal strain.
As an alternative to direct growth, wafer bonding allows the homogeneous epitaxial growth of different high performance solar cells on different substrate wafers. Once the wafers are bonded, one wafer can be lifted off by wet chemicals. The direct bonding of GaInP/GaAs on GaAs and GaInAsP/GaInAs on InP has yielded four junction solar cells with world record efficiency for concentration applications. (See, F. Dimroth, M. Grave, P. Beutel, U. Fiedeler, C. Karcher, T. N. D. Tibbits, E. Oliva, G. Siefer, M. Schachtner, A. Wekkeli, A. W. Bett, R. Krause, M. Piccin, N. Blanc, C. Drazek, E. Guiot, B. Ghyselen, T. Salvetat, A. Tauzin, T. Signamarcheix, A. Dobrich, T. Hannappel, K. Schwarzburg, Prog. Photovolt. Res. Appl. 2014, 22, 277.) There is also ongoing research on the bonding of III-V semiconductors to Si for solar cells, which mainly focuses on low temperature, interface optical transparency, and thermal and electrical conductivity. (See, J. Liang, S. Nishida, M. Morimoto, N. Shigekawa, Electron. Lett. 2013, 49, 830; J. Liang, T. Miyazaki, M. Morimoto, S. Nishida, N. Watanabe, N. Shigekawa, Appl. Phys. Express 2013, 6, 021801 and S. Essig, O. Moutanabbir, A. Wekkeli, H. Nahme, E. Oliva, A. W. Bett, F. Dimroth, J. Appl. Phys. 2013, 113, 203512.) Surface-activated direct bonding at room temperature avoids high temperature annealing, and a GaInP/GaAs/Si triple junction solar cell has been demonstrated. (See, K. Derendorf, S. Essig, E. Oliva, V. Klinger, T. Roesener, S. P. Philipps, J. Benick, M. Hermle, M. Schachtner, G. Siefer, W. Jager, F. Dimroth, IEEE J. Photovolt. 2013, 3, 1423.) Direct fusion bonding under a low post-bonding temperature of 500° C. or less has also been investigated and a dual-junction AlGaAs/Si has been fabricated. (See, K. Tanabe, K. Watanabe, Y. Arakawa, Sci. Rep. 2012, 2, DOI 10.1038/srep00349.) Non-direct bonding, which uses agents, such as metal or carbon nanotubes, would sacrifice the interface properties and is thus not optimal for photovoltaic applications. (See, C.-T. Lin, W. E. McMahon, J. S. Ward, J. F. Geisz, M. W. Wanlass, J. J. Carapella, W. Olavarria, E. E. Perl, M. Young, M. A. Steiner, R. M. France, A. E. Kibbler, A. Duda, T. E. Moriarty, D. J. Friedman, J. E. Bowers, Prog. Photovolt. Res. Appl. 2014, n/a and A. Boca, J. C. Boisvert, D. C. Law, S. Mesropian, N. H. Karam, W. D. Hong, R. L. Woo, D. M. Bhusari, E. Turevskaya, P. Mack, P. Glatkowski, in 2010 35th IEEE Photovolt. Spec. Conf. PVSC, 2010, pp. 003310-003315.) In all of the above-mentioned wafer bonding processing techniques, the bonded III-V wafers are usually removed by chemical etching after the bonding, in order to expose the top solar cell layer. This is not only a waste of expensive materials, but also results in high production costs.
Recently, with As2Se3 as the bonding agent, triple junction InGaP/GaAs/InGaAsNSb epitaxially-grown solar cells have been bonded onto Ge solar cells after being released from the GaAs wafer. (See, X. Sheng, C. A. Bower, S. Bonafede, J. W. Wilson, B. Fisher, M. Meitl, H. Yuen, S. Wang, L. Shen, A. R. Banks, C. J. Corcoran, R. G. Nuzzo, S. Burroughs, J. A. Rogers, Nat. Mater. 2014, 13, 593.)